Ionization cross sections for ion-atom collisions in high energy ion beams

Knowledge of ion-atom ionization cross sections is of great importance for many accelerator applications. When experimental data and theoretical calculations are not available, approximate formulas are frequently used. Based on experimental data and theoretical predictions, a new fit for ionization cross sections by fully stripped ions is proposed. The Born approximation and classical trajectory calculations are frequently used to estimate the cross sections. Neither approximation is expected to be valid over the entire range of projectile ions and target atoms. Aspects of both models must be included in order to address the short-comings in the underlying assumptions. A large difference in cross section, up to a factor of six, calculated in quantum mechanics and classical mechanics, has been obtained for 3.2 GeV I^- and Cs⁺ ions. Because at such high velocities the Born approximation is well validated, the classical trajectory approach fails to correctly predict the stripping cross section at high energies for electron orbitals with low ionization potential.